1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an alignment layer for initial alignment of liquid crystal in an LCD device.
2. Discussion of the Related Art
Among various ultra-thin flat type display devices, which include a display screen having a thickness of several centimeters, a liquid crystal display (LCD) device has great attention because of its wide variety of uses, including notebook computers, monitors, spacecraft, aircraft, and etc.
Generally, the LCD device includes a color filter substrate having a color filter layer, a thin film transistor substrate having a thin film transistor, and a liquid crystal layer formed between the color filter substrate and the thin film transistor substrate. At this time, the thin film transistor substrate is positioned opposite to the color filter substrate.
In the LCD device, an alignment direction of the liquid crystal layer is changed according to application of voltage, thereby controlling the transmittance of light. Accordingly, images are displayed in the LCD device. For application of voltage, electrodes are formed on the thin film transistor substrate and the color filter substrate. That is, a pixel electrode is formed on the thin film transistor substrate, and a common electrode is formed on the color filter substrate, whereby an electric field is vertically formed between the thin film transistor substrate and the color filter substrate (for example, Twisted Nematic (TN) mode). In another method, the pixel electrode and the common electrode may be foamed on the thin film transistor substrate, thereby forming an electric field parallel to the two substrates (for example, In-Plane Switching (IPS) mode).
FIG. 1 is an exploded perspective view of a TN mode LCD device according to the related art. As shown in FIG. 1, a thin film transistor substrate 10 of the TN mode LCD device according to the related art includes a gate line 12, a data line 14, a thin film transistor T, and a pixel electrode 16. At this time, the thin film transistor T is formed at a crossing of the gate and data lines 12 and 14, and the pixel electrode 16 is connected with the thin film transistor T. Also, a color filter substrate 20 includes a black matrix layer 22 including black matrix patterns for prevention of light leakage, a R/G/B color filter layer 24 having red, green and blue color patterns, each color pattern provided between the black matrix patterns, and a common electrode 25 formed on the R/G/B color filter layer 24. In this case, an electric field is vertically formed between the pixel electrode 16 of the thin film transistor substrate 10 and the common electrode 25 of the color filter substrate 20, thereby controlling the alignment direction of liquid crystal.
Thereafter, the substrates 10 and 20 are attached to each other to form one liquid crystal panel in which a liquid crystal layer is formed between the substrates 10 and 20.
In the meantime, when forming the liquid crystal layer between the two substrates 10 and 20, regular alignment of liquid crystal molecules is required. For this, although not shown, alignment layers are provided on the thin film transistor substrate 10 and the color filter substrate 20 for initial alignment of liquid crystal.
The alignment layer for initial alignment of liquid crystal is generally formed in a rubbing alignment method.
The rubbing alignment method includes steps of coating a thin filming of an organic polymer such as polyimide on a substrate, aligning a side chain of the organic polymer to a predetermined direction by rubbing the coated organic polymer by rotating a rubbing roll coated with rubbing cloth, and curing the aligned organic polymer.
Accordingly, the liquid crystal is aligned according to the aligned direction of the side chain of the organic polymer. That is, the moving direction of the rubbing roll corresponds to the alignment direction of liquid crystal.
However, the rubbing alignment method has the following disadvantages.
First, when the rubbing cloth is irregular, there may be resulting light leakage because of improper or no alignment of the liquid crystal.
FIG. 2 is a perspective view of illustrating the problem generated by the irregular rubbing cloth.
As illustrated in FIG. 2, the elements such as the thin film transistor, the color filter layer and the electrode layer are formed on the substrate. Thus, when the rubbing roll 30 coated with the rubbing cloth 32 rotates on the elements formed on the substrate 10 or 20, some portion 32a of the rubbing cloth 32 may be irregular. As a result, the side chain of the organic polymer on the portion of the substrate rubbed with the irregular portion 32a of the rubbing cloth 32 is not aligned or not aligned properly. Therefore, there may be the light leakage due to the irregular alignment of liquid crystal.
Second, when the rubbing cloth does not in contact with the substrate, there may be light leakage.
FIG. 3 is a perspective view illustrating the alignment state of liquid crystal when the rubbing cloth is not in contact with the substrate.
As explained earlier, electrode layers, such as pixel electrodes and a common electrode, are formed on a substrate. Due to a step height in electrode layers formed on a substrate 10, as illustrated in FIG. 3, a region (region “A”) is formed where a rubbing cloth 32 does not come into contact with the substrate 10. In this case, the alignment of a liquid crystal is not uniform in the region (“A”), resulting in light leakage.
In conclusion, according to a related art rubbing alignment method, when the arrangement of a rubbing cloth is non-uniform or a rubbing cloth does not come into contact with a substrate, rubbing cannot be performed well, causing the problem of light leakage. Thus, there is a need for a novel liquid crystal alignment method to solve the problems of the related art rubbing alignment method.
The above-mentioned problems of the related art rubbing alignment method are attributed to physical contact between a rubbing roll and a substrate.